Talks

Roger Colbeck's talk at QCRYPT 2012 on our joint work with Jonathan Barrett
on memory attacks on device-independent quantum cryptography is
here.

Commentaries
The MIT Technology Review blog's
report on the above work.
Note our author correction letter published below the
original article. (The originally posted article
wrongly suggested our attacks apply
to all types of quantum cryptography rather than specifically to
device-independent implementations by parties who cannot rely on
any property of the quantum devices they are using).

Many Worlds Quantum Theory and its problems

Our book

Many Worlds? Everett, Quantum Theory, and Reality was published
by Oxford University Press in June 2010.

Unlike the other editors, I'm sceptical about whether
many-worlds quantum theory can actually be made into a well-defined
and scientifically useful theory, and one of my contributions
to the book is the question mark in the title.

Another is my chapter
One World Versus Many, which includes
an extended critique of recent
attempts to make sense of Everett's many-worlds ideas.

Synopsis

What would it mean to apply quantum theory, without
restriction and without involving any notion of measurement
and state reduction, to the whole universe? What would
realism about the quantum state then imply?

This book brings together an illustrious team of
philosophers and physicists to debate these questions. The
contributors broadly agree on the need, or aspiration, for a
realist theory that unites micro- and macro-worlds. But they
disagree on what this implies. Some argue that if unitary
quantum evolution has unrestricted application, and if the
quantum state is taken to be something physically real, then
this universe emerges from the quantum state as one of
countless others, constantly branching in time, all of which
are real. The result, they argue, is many worlds quantum
theory, also known as the Everett interpretation of quantum
mechanics. No other realist interpretation of unitary quantum
theory has ever been found.

Others argue in reply that this picture of many worlds is in no
sense inherent to quantum theory, or fails to make physical sense, or
is scientifically inadequate. The stuff of these worlds, what they are
made of, is never adequately explained, nor are the worlds precisely
defined; ordinary ideas about time and identity over time are
compromised; no satisfactory role or substitute for probability can be
found in many worlds theories; they can't explain experimental data;
anyway, there are attractive realist alternatives to many worlds.

Twenty original essays, accompanied by commentaries and
discussions, examine these claims and counterclaims in depth. They
consider questions of ontology - the existence of worlds; probability
- whether and how probability can be related to the branching
structure of the quantum state; alternatives to many worlds - whether
there are one-world realist interpretations of quantum theory that
leave quantum dynamics unchanged; and open questions even given many
worlds, including the multiverse concept as it has arisen elsewhere in
modern cosmology. A comprehensive introduction lays out the main
arguments of the book, which provides a state-of-the-art guide to many
worlds quantum theory and its problems.

Related Experimental Work

The Geneva group carried out a beautiful experiment
aiming to close the collapse locality loophole in Bell experiments,
described in the last paper above. In their experiment
the outcomes of Bell measurements were, for the first time,
macroscopically recorded in space-like
separated regions by fast-moving piezocrystals.
"Macroscopically" here means that matter distributions were
altered in such a way that the gravitational fields
corresponding to distinct outcomes are (according to guesstimates
due to Penrose and Diosi) distinguishable.
The experiment is described
here .

Popular Articles

I wrote a popular account of the problems in reconciling quantum theory with a
scientific account of reality, and hence with the rest of science, for
Aeon magazine (published in January 2014):
Our Quantum Reality Problem

Speculative Exobiology and the Fermi Problem

Papers

Commentaries

My earlier papers on
these subjects include a
classification
of the unitary highest weight representations of the Virasoro, Ramond and
Neveu-Schwarz algebras, which uses the so-called
GKO construction (also known as the
coset construction),
which relates highest weight representations of
these algebras to those of affine Kac-Moody algebras.

These results are central to understanding two-dimensional conformal
field theories, which describe the scaling behaviour of a large class
of two-dimensional systems at criticality. At the critical point,
lattice models, and the physical systems they represent, have a
fractal-like structure and become scale invariant.
Here is an example of an Ising model critical state at various scales:

Because the physics is local, the models actually display local
scale invariance or conformal invariance, which in two dimensions
is a very rich symmetry, represented in field theory
by the action of an infinite dimensional Lie algebra, the
Virasoro algebra.
The
unitary classification of Virasoro algebra highest weight representations
explains
the previously puzzling appearance of particular
simple rational numbers as critical exponents for the Ising
model, tricritical Ising model, 3-state Potts model, tricritical
3-state Potts model, and an infinite series of two dimensional
lattice models, several of which describe the critical behaviour
of naturally occurring two dimensional systems.
The unitary classification of Ramond and Neveu-Schwarz algebra
highest weight representations highlights the naturally occurring
supersymmetry occurring in two dimensional systems described
by the tricritical Ising model and a further infinite series of
models.

Some results on the representation theory of N=2 superconformal
algebras, which also describe naturally occurring two dimensional
systems (and have applications in string theory)
are
here.

My other work on the representation theory of the Virasoro
algebra includes descriptions of its
singular vectors
(see also
here)
and a recursion formula for the
signature characters
of its highest weight representations.
The technique for calculating signature characters gives an
alternative way of characterising unitary
highest weight representations of Lie algebras: some calculations
for simple Lie algebras are
here.